As I understand it most of these airframes aren't fit to fly with a crew anymore because they've aged out. Airframes that don't get into this program get scrapped.

Indeed, the F16 was originally designed for about 8 000 hours of service time. And most airframes appear to have lasted about 20 years.

Once your aircraft gets to the point it will start to fail, why not use it as a few as a target?

They are still pushing the airframe through 12k hours.

And already tested through 15k+ hours I think.

They are shot at with missiles, but are rarely every 'shot down'. They can use fully capable missiles without warheads. A "kill" is getting the missile to a point that it would detonate, which likely means flying very close but not actually touching the target. You don't use manned craft, but you still expect the target to survive most encounters.

Probably used not for shooting at, but rather for detection, tracking and such. Being on par or a little better than upcoming adversary systems works best for this.

I'd rather have the military test their capabilities in production, than have them wait for a war to see if everything works.